CD1 molecules are a family of highly conserved antigen presenting proteins that are similar in function to classical MHC molecules. While classical MEC molecules present peptides, CD1 proteins bind and display a variety of lipids and glycolipids to T lymphocytes. The five known isoforms are classified into two groups, group I (CD1a, CD1b, CD1c, and CD1e in humans) and group II (CD1d in humans and mice) based on similarities between nucleotide and amino acid sequences.
A great diversity of lipids and glycolipids has been shown to bind specifically to each of the four isoforms. The first crystal structure of murine (m)CD1d revealed that it adopts a folded conformation closely related to major histocompatibility complex (MHC) class I proteins. It further revealed that mCD1d could accommodate long lipid tails in two hydrophobic pockets, designated A′ and F′, located in the binding groove. Two additional structures of hCD1b and hCD1a confirmed this model by demonstrating that CD1, when loaded with antigenic glycolipids, binds the lipid portion in a hydrophobic groove while making available the hydrophilic sugar moiety to make contact with the T-cell receptor.
Mammalian and mycobacterial lipids are known to be presented by human CD1a, CD1b, CD1c and CD1d [Porcelli, S. A. & Modlin, R. L. (1999) Annu. Rev. Immunol. 17, 297-329] α-GalCer, a lipid found in the marine sponge Agelas mauritianus, has been, to date, the most extensively studied ligand for CD1d. α-GalCer, when bound to CD1d, stimulates rapid Th1 and Th2 cytokine production by Vα14i natural killer T (Vα14i NKT) cells in mice and the human homologue Vα24i NKT cells. However, its physiological significance in mammals remains unclear as it is enigmatic why an α-galactosyl ceramide of marine origin is such a potent agonist Other known ligands, such as a bacterial phospholipid (PIM4), a tumor derived ganglioside GD3, a C-linked analog of α-GalCer, α-GalCer analogs with different lipid chain lengths and a phosphoethanolamine, found in human tumor extracts, stimulate only relatively small populations of CD1d-restricted NKT cells.
Natural Killer (NK) cells typically comprise approximately 10 to 15% of the mononuclear cell fraction in normal peripheral blood. Historically, NK cells were first identified by their ability to lyse certain tumor cells without prior immunization or activation. NK cells also serve a critical role in cytokine production, which may be involved in controlling cancer, infection and possibly in fetal implantation.